Cell membrane and Related Topics

Onion (Allium cepa) root cells in different phases of the cell cycle (drawn by E. B. Wilson, 1900)

Cell (biology)

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Basic structural and functional unit of life forms.

Detailed diagram of lipid bilayer of cell membrane

A fluorescent image of an endothelial cell. Nuclei are stained blue, mitochondria are stained red, and microfilaments are stained green.

Human cancer cells, specifically HeLa cells, with DNA stained blue. The central and rightmost cell are in interphase, so their DNA is diffuse and the entire nuclei are labelled. The cell on the left is going through mitosis and its chromosomes have condensed.

Prokaryotes divide by binary fission, while eukaryotes divide by mitosis or meiosis.

An outline of the catabolism of proteins, carbohydrates and fats

An overview of protein synthesis.
Within the nucleus of the cell (light blue), genes (DNA, dark blue) are transcribed into RNA. This RNA is then subject to post-transcriptional modification and control, resulting in a mature mRNA (red) that is then transported out of the nucleus and into the cytoplasm (peach), where it undergoes translation into a protein. mRNA is translated by ribosomes (purple) that match the three-base codons of the mRNA to the three-base anti-codons of the appropriate tRNA. Newly synthesized proteins (black) are often further modified, such as by binding to an effector molecule (orange), to become fully active.

Staining of a Caenorhabditis elegans highlights the nuclei of its cells.

Stromatolites are left behind by cyanobacteria, also called blue-green algae. They are the oldest known fossils of life on Earth. This one-billion-year-old fossil is from Glacier National Park in the United States.

Robert Hooke's drawing of cells in cork, 1665

Every cell consists of a cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids.

Lipid bilayer

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Thin polar membrane made of two layers of lipid molecules.

The three main structures phospholipids form in solution; the liposome (a closed bilayer), the micelle and the bilayer.

TEM image of a bacterium. The furry appearance on the outside is due to a coat of long-chain sugars attached to the cell membrane. This coating helps trap water to prevent the bacterium from becoming dehydrated.

Diagram showing the effect of unsaturated lipids on a bilayer. The lipids with an unsaturated tail (blue) disrupt the packing of those with only saturated tails (black). The resulting bilayer has more free space and is, as a consequence, more permeable to water and other small molecules.

Illustration of a GPCR signaling protein. In response to a molecule such as a hormone binding to the exterior domain (blue) the GPCR changes shape and catalyzes a chemical reaction on the interior domain (red). The gray feature is the surrounding bilayer.

Transmission Electron Microscope (TEM) image of a lipid vesicle. The two dark bands around the edge are the two leaflets of the bilayer. Historically, similar images confirmed that the cell membrane is a bilayer

Human red blood cells viewed through a fluorescence microscope. The cell membrane has been stained with a fluorescent dye. Scale bar is 20μm.

3d-Adapted AFM images showing formation of transmembrane pores (holes) in supported lipid bilayer

Illustration of a typical AFM scan of a supported lipid bilayer. The pits are defects in the bilayer, exposing the smooth surface of the substrate underneath.

Structure of a potassium ion channel. The alpha helices penetrate the bilayer (boundaries indicated by red and blue lines), opening a hole through which potassium ions can flow

Schematic illustration of pinocytosis, a type of endocytosis

Exocytosis of outer membrane vesicles (MV) liberated from inflated periplasmic pockets (p) on surface of human Salmonella 3,10:r:- pathogens docking on plasma membrane of macrophage cells (M) in chicken ileum, for host-pathogen signaling in vivo.

Schematic showing two possible conformations of the lipids at the edge of a pore. In the top image the lipids have not rearranged, so the pore wall is hydrophobic. In the bottom image some of the lipid heads have bent over, so the pore wall is hydrophilic.

Illustration of lipid vesicles fusing showing two possible outcomes: hemifusion and full fusion. In hemifusion, only the outer bilayer leaflets mix. In full fusion both leaflets as well as the internal contents mix.

Schematic illustration of the process of fusion through stalk formation.

Diagram of the action of SNARE proteins docking a vesicle for exocytosis. Complementary versions of the protein on the vesicle and the target membrane bind and wrap around each other, drawing the two bilayers close together in the process.

The cell membranes of almost all organisms and many viruses are made of a lipid bilayer, as are the nuclear membrane surrounding the cell nucleus, and membranes of the membrane-bound organelles in the cell.

Bacteria

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Bacteria (singular bacterium, common noun bacteria) are ubiquitous, mostly free-living organisms often consisting of one biological cell.

Phylogenetic tree of Bacteria, Archaea and Eucarya. The vertical line at bottom represents the last universal common ancestor.

Bacteria display many cell morphologies and arrangements

The range of sizes shown by prokaryotes (Bacteria), relative to those of other organisms and biomolecules.

Structure and contents of a typical Gram-positive bacterial cell (seen by the fact that only one cell membrane is present).

An electron micrograph of Halothiobacillus neapolitanus cells with carboxysomes inside, with arrows highlighting visible carboxysomes. Scale bars indicate 100 nm.

Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface

Bacillus anthracis (stained purple) growing in cerebrospinal fluid

Many bacteria reproduce through binary fission, which is compared to mitosis and meiosis in this image.

Helium ion microscopy image showing T4 phage infecting E. coli. Some of the attached phage have contracted tails indicating that they have injected their DNA into the host. The bacterial cells are ~ 0.5 µm wide.

Transmission electron micrograph of Desulfovibrio vulgaris showing a single flagellum at one end of the cell. Scale bar is 0.5 micrometers long.

The different arrangements of bacterial flagella: A-Monotrichous; B-Lophotrichous; C-Amphitrichous; D-Peritrichous

Streptococcus mutans visualised with a Gram stain.

Phylogenetic tree showing the diversity of bacteria, compared to other organisms. Here bacteria are represented by three main supergroups: the CPR ultramicrobacterias, Terrabacteria and Gracilicutes according to recent genomic analyzes (2019).

Overview of bacterial infections and main species involved.

Colour-enhanced scanning electron micrograph showing Salmonella typhimurium (red) invading cultured human cells

In bacterial vaginosis, beneficial bacteria in the vagina (top) are displaced by pathogens (bottom). Gram stain.

Antonie van Leeuwenhoek, the first microbiologist and the first person to observe bacteria using a microscope.

Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles.

Cell wall

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Cell wall in multicellular plants – its different layers and their placement with respect to protoplasm (highly diagrammatic)

Molecular structure of the primary cell wall in plants

Photomicrograph of onion root cells, showing the centrifugal development of new cell walls (phragmoplast)

Chemical structure of a unit from a chitin polymer chain

Scanning electron micrographs of diatoms showing the external appearance of the cell wall

Illustration of a typical gram-positive bacterium. The cell envelope comprises a plasma membrane, seen here in light brown, and a thick peptidoglycan-containing cell wall (the purple layer). No outer lipid membrane is present, as would be the case in gram-negative bacteria. The red layer, known as the capsule, is distinct from the cell envelope.

A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane.

Cholesterol

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Any of a class of certain organic molecules called lipids.

Lipid logistics: transport of triglycerides and cholesterol in organisms in form of lipoproteins as chylomicrons, VLDL, LDL, IDL, HDL.

Cholesterolemia and mortality for men and women 60 years

Reference ranges for blood tests, showing usual, as well as optimal, levels of HDL, LDL, and total cholesterol in mass and molar concentrations, is found in orange color at right, that is, among the blood constituents with the highest concentration.

Steroidogenesis, using cholesterol as building material

Space-filling model of the Cholesterol molecule

Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell membranes.

Cytoskeleton

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Complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, excluding bacteria and archaea.

It extends from the cell nucleus to the cell membrane and is composed of similar proteins in the various organisms.

Cross-sectional view of the structures that can be formed by phospholipids in an aqueous solution

Biological membrane

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Selectively permeable membrane that separates cell from the external environment or creates intracellular compartments.

A fluid membrane model of the phospholipid bilayer.

Biological membranes, in the form of eukaryotic cell membranes, consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions.

Proteins in different cellular compartments and structures tagged with green fluorescent protein

Cytoplasm

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In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus.

Scheme of a liposome formed by phospholipids in an aqueous solution.

Vesicle (biology and chemistry)

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Structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer.

Electron micrograph of a cell containing a food vacuole (fv) and transport vacuole (tv) in a malaria parasite.

The membrane enclosing the vesicle is also a lamellar phase, similar to that of the plasma membrane, and intracellular vesicles can fuse with the plasma membrane to release their contents outside the cell.